As this type of piston, there is known, for example, a piston of Japanese Patent Application Publication No. 2008-008170 (referred to as “Patent Document” hereinafter) in which a crown part is configured by a flat base surface and a ring-shaped bulging part bulging above the base surface.
The weight of this piston is reduced by the formation of a recess on a lower surface of a land part of the piston, and disruption of the weight balance of the piston is prevented by providing this recess with two ribs of different weights. Two seal ring grooves and one oil ring groove are formed on an outer circumferential surface of the land part, successively from the top. Seal rings are fitted into the seal ring grooves in order to keep air-tightness of a combustion chamber. An oil ring is fitted into the oil ring groove in order to scrape oil off an inner circumferential surface of a cylinder.
By the way, in a state in which the piston is inserted and fitted into the cylinder, an extremely narrow annular concave space is formed between the top seal ring (referred to as “top ring” hereinafter), the inner circumferential surface of the cylinder, and the outer circumferential surface of the land part. Intake air-fuel mixture cannot combust easily in this concave space, leaving unburnt gas therein. Therefore, from the perspective of inhibiting the generation of HC (hydrocarbon), it is preferred that the size of the concave space be minimized. In order to do so, the base surface needs to be brought as close as possible to the position of the top ring; however, the closer the base surface is to the position of the top ring, the more difficult for the top ring to tolerate the heat of combustion. This is not preferable in terms of thermally protecting the top ring.